function showHint(str)
{
    if (str.length==0){ 
        document.getElementById("txtHint").innerHTML="";
        return;
    }
    if (window.XMLHttpRequest){// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else{// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function(){
        if (xmlhttp.readyState==4 && xmlhttp.status==200){
            document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET","../script/checkUserAvailable.php?user="+str,true);
    xmlhttp.send();
}
